You might have heard about the placebo effect: Some people get well even though they are given a dummy pill that consists simply of starch and sugar.
Unfortunately, the positive effect has an evil twin: The nocebo effect. This causes people to experience symptoms even if given an inactive pill. They are so convinced they are given medicine with serious side-effects that a sugar pill will also cause symptoms.
Both effects show the fantastic power of our minds. We can think ourselves well, and we can think ourselves sick. Be careful to tell your mind you are well or getting better. It helps.